1. Field of the Invention
The present invention relates to a process for joining or riveting two or more pieces of lapped metal together. The method allows a range of non-ferrous and ferrous metals to be joined, e.g., aluminum, magnesium, copper, titanium, iron, and their respective alloys. More particularly, the invention represents an alternative process for riveting two or more aluminum alloy products together.
2. Prior Art
Conventional solid-phase welding (friction welding) involves rubbing two surfaces together under pressure in relative motion for sufficient time until metal between the two surfaces becomes thermally softened and in a plastic state. As shown in FIG. 1a, friction welding commonly involves rotating a first component A under pressure against a second component B. Alternatively, the component A may be inserted into a bore defined in the component B and rotated to produce a joint within the bore. A more recent development is referred to as xe2x80x9cfriction plunge weldingxe2x80x9d which International Patent Classifications B23k 20/12 and B29c 65/06 on xe2x80x9cImprovements Relating to Friction Weldingxe2x80x9d, describe as being xe2x80x9ca method of operating on a work piece, that method comprising offering a probe of material harder than the work piece material to a continuous or substantially continuous or substantially continuous surface of the work piece; causing relative cyclic movement between the probe and the work piece while urging the probe and the work piece together whereby frictional heat is generated as the probe enters the work piece so as to create a plasticized region in the work piece material around the probe; stopping the relative cyclic movement; and allowing the plasticized material to solidify around the probe.xe2x80x9d As shown in FIG. 1b, conventional friction plunge welding involves immersing a relatively hard material H into a relatively soft material S with different metal combinations, e.g., steel into aluminum, copper into aluminum, and the like as described in Connect, September 1993.
Other mechanisms for joining two or more lapped plates include friction hydro pillar processing (FHPP) and friction taper stud welding (FTSW). Each of FHPP and FTSW are employed with a predrilled hole having a diameter larger than that of the rivet material for FHPP and one using a tapered drill hole for FTSW. These conventional spot-based mechanical fastening processes entail one or more of the following elements: (1) making holes through the parts to be joined as with all riveting processes (pop rivets, self-piercing rivets, xe2x80x9cblindxe2x80x9d rivets); (2) an absence of metallurgical bonding between the joint parts which makes fastening fully dependent on mechanical locking; and/or (3) a pronounced deformation of the parts being joined (e.g., self-piercing rivets and clinching). Mechanical fastening is also expensive, prone to seepage of environmental elements (salt water, condensation, and the like) and often loosens over time. Loosening of fastened joints may compromise the service performance of the joined components.
Accordingly, a need remains for a method of joining or riveting two or more pieces of lapped metal together wherein the metals may be the same or different and wherein the rivet used to join the metal pieces together is not necessarily different from the metals being joined.
This need is met by the method of the present invention which was conceived by realizing that it is possible to force-plunge, pierce, penetrate into and metallurgically bond two or more metal parts lapped or stacked together (xe2x80x9cstack upsxe2x80x9d), by striking a balance between (a) a rivet geometry (i.e., tip shape and diameter and included angle), (b) the strength or hardness of rivets and parts being joined before and during friction welding, (c) the melting temperature range of rivets and the parts to be joined, (d) the respective thicknesses of joined parts, (e) the rate of heat dissipation into the parts and rivets through conduction, and (f) other friction welding parameters including forging and welding force, bum off, revolutions per minute, plunge rate and the like, all which affect heat generation and the forces experienced in a given joining region (i.e., between the rivets and the parts to be joined). While the present invention is particularly suited for joining metal having no predrilled holes or apertures, such holes not being required herein, it is to be understood that the presence of a partially formed hole or a fully formed hole through at least one of the metal parts being so joined may be beneficial in increasing the rate of completion of the method.
The present invention of friction plunge riveting differs from conventional uses of friction plunge welding which require plunging a significantly harder material into a significantly softer material (e.g., copper or steel into aluminum). The friction plunge riveting process of the present invention substantially provides a more homogenous joint region in which the constituent elements of the rivet and the work piece are made from the same metal families. For example, two or more aluminum alloy parts (one or more of which may be substantially pure aluminum) may be joined with an aluminum alloy rivet. There is no requirement for an overlap within the same sub-family of alloys. As one representative example of an interfamily relationship of riveting according to the present invention, components of Aluminum Association Series (AA) 5xxx alloy may be joined with and AA 7xxx alloy rivet. Preferably, however, both the work piece materials and the rivet join a work piece together should have about 50% or greater commonality (or overlap) in the major alloying components. The present invention differs from friction plunge welding in that the probe or rivet used in friction plunge riveting can become partially plasticized as such, friction plunge riveting is particularly suited for applications which require the joining of two or more lapped plates. In such situations, the rivet material may constitute essentially the same or substantially similar material as the work pieces being joined or riveted together. For example, friction plunge riveting of the present invention allows for plunging or piercing aluminum alloy rivets into an aluminum alloy or substantially pure aluminum, copper alloy rivets into parts made of copper alloys or pure copper, magnesium alloy rivets into a magnesium alloy or pure magnesium component parts, titanium alloy rivets into a titanium alloy or pure titanium parts, or steel rivets into steel parts.
In contrast to the conventional spot-based mechanical fastening, friction plunge riveting according to the present invention relies on a metallurgical bond formed between the rivet and the parts being joined. The riveting process of the present invention thus a) eliminates the need to machine a hole in the parts being joined, b) effects a full metallurgical bond between the rivet and the parts being joined, and c) minimizes deformation of the parts and/or the rivet unless the deformation is designed for aesthetic reasons.
A complete understanding of the invention will be obtained from the following description when taken in connection with the accompanying drawing figures wherein like reference characters identify like parts throughout.